1. Field of the Invention
The present invention relates to a masterbatch pellet mixture. Priority is claimed on Japanese Patent Application No. 2003-125095, filed Apr. 30, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
When compounding agents such as dyes, pigments or other additives are added to and mixed with a resin material, a method is used wherein first the entire quantity of the compounding agent is added to a small quantity of the resin material, and is carefully mixed and dispersed to generate a high concentration material, and the majority of the resin material is then added to this high concentration material to generate a product with the target concentration. This method offers certain advantages including the fact that the dispersion and mixing is completed within a short time period, and the fact that the process can be managed with ease. This high concentration compound material is called a masterbatch. In those cases where a thermoplastic resin is used as the aforementioned resin material, pellet-like masterbatches, which are produced by adding a high concentration of a compounding agent to the thermoplastic resin and then mixing to generate a uniform mixture, are already known. In the present invention, the resin used to adjust the masterbatch to the target concentration is termed the additional resin.
In a typical method of using masterbatch pellets, the masterbatch pellets and the additional resin pellets are first mixed together in a mixing device such as a tumbler mixer, and the resulting mixture is then supplied to a molding apparatus.
Examples of methods of supplying the aforementioned mixture of the masterbatch pellets and the additional resin pellets to the molding apparatus include free fall methods and air blow feeding methods. The problem that arises is that during supply of the pellet mixture to the molding apparatus, the pellet mixture separates and become non-uniform, depending on factors such as the transport distance and the method used, and as a result, the molded product develops compositional irregularities, making it impossible to produce a product with uniform coloring and functionality. Conventionally, the method typically employed to prevent this type of separation of the different pellets is that all the pellets are of substantially the same shape and size.
Furthermore, another method used for preventing separation is the method described below. This method uses a separate supply system wherein the masterbatch pellets and the additional resin pellets are not premixed, but are rather supplied separately to the molding apparatus using separate supply devices, and the masterbatch pellets and the additional resin pellets are then mixed immediately prior to entry into the material supply port of the molding apparatus. In this method, because the mixing of the masterbatch pellets and the additional resin pellets is conducted immediately prior to entry into the material supply port of the molding apparatus, separation of the pellets can be suppressed.
However, in recent years, demands for high functionality of molded products has increased significantly, resulting in a large increase in the number of different masterbatches containing specified additives. Furthermore, molded products of all different hues are now demanded, and therefore the number of different masterbatches containing the dyes or additives required for generating those different hues is also increasing. In the separate supply system described above, a separate supply device must be provided for each different masterbatch, and consequently the equipment costs associated with assembling a production line have become extremely expensive. The only way in which to reduce these equipment costs is to reduce the number of supply devices, and consequently the first method described above must be used, wherein a plurality of masterbatch pellets and additional resin pellets are mixed uniformly in a mixer, and this pellet mixture is then supplied to the molding apparatus.
As described above, if a number of different masterbatch pellets are mixed together, then a problem arises in that differences in the shape, size and specific gravity of the different masterbatch pellets, and/or with electrostatic effects, cause the mixture to undergo separation into a non-uniform mixture, with like pellets grouping together. This separation of different masterbatch pellets is particularly marked in those cases where masterbatch pellets with a large difference in specific gravity are mixed together. Pellet separation caused by differences in shape or electrostatic effects can be suppressed to some degree by a method such as regulating the shape and size of the pellets, and adding antistatic agents. However, the specific gravity of any particular type of masterbatch pellets varies depending on the specific additives, dyes or pigments added to the pellets. Accordingly, adjusting the specific gravity of a number of different masterbatch pellets to the same value is problematic.
In order to resolve this problem, a method has been disclosed in which the weight ratio (Wb/Wa) of the average weight (Wb) of a single masterbatch pellet relative to the average weight (Wa) of a single additional resin pellet is set to a value within a range from 0.45 to 0.95 (for example, see Japanese Unexamined Patent Application, First Publication No. Hei 7-102155). However in this method, the average weight of each of the pellets must be set within a specific range, and in order to match the average weights, high specific gravity pellets must be reduced in size, and low specific gravity pellets must be increased in size. Because this leads to the situation where pellets of different specific gravity have different sizes, separation of the pellets can not be adequately prevented. High specific gravity pellets are particularly problematic as their small size means that pellet separation is more marked.
Furthermore, a masterbatch pellet mixture formed from a number of different masterbatch pellets in which the difference between the maximum and minimum apparent specific gravity values of pellets within the mixture is suppressed to no more than 0.5 g/cm3 has also been proposed. In order to adjust the apparent specific gravity values in this manner, a method is disclosed in which a foaming agent or a high specific gravity filler is added to each of the different pellets (for example, see Japanese Unexamined Patent Application, First Publication No. Hei 7-216099). However in this method, in those cases where a foaming agent is added to the masterbatch pellets, molding problems can develop, including the appearance of silver streaking on molded product surfaces (a phenomenon wherein residual gas from the foaming agent occupies space between the molded article and the mold itself, causing streak shaped areas of surface roughness on the surface of the molded product) or the generation of holes in molded films. In contrast, in those cases where a high specific gravity filler is added to the masterbatch pellets, because the high specific gravity filler often comprises a heavy metal, the number of materials that can be added as a high specific gravity filler is reasonably limited due to environmental concerns. Furthermore, the addition of these foaming agents or high specific gravity fillers also affects the hue of the molded product, which is not ideal.
In addition, a method has been disclosed in which the masterbatch pellets and the additional resin pellets are supplied directly, using a quantitative supply device, to the upper section of a screw in the molding apparatus, without undergoing premixing in a blender or mixer (for example, see Japanese Unexamined Patent Application, First Publication No. Hei 11-279282). However in order to ensure uniform supply of the pellets to the molding apparatus, the pellets used in this method must have a shape and size that falls within a specific range, and the difference in specific gravity between pellets must also be minimized. In particular, masterbatch pellets containing added titanium oxide, which has a very large specific gravity, must be restricted to a titanium oxide content within the masterbatch of about no more than 50% by mass in order to enable the difference in specific gravity values between pellets to be small, meaning a loss of freedom in terms of the range of possible compositions.
An object of the present invention is to provide a masterbatch pellet mixture, which when supplied to a molding apparatus, maintains a uniform mixed state and enables stable supply with no separation of the different pellets, even if the masterbatch pellet mixture comprises a mixture of high specific gravity masterbatch pellets and low specific gravity masterbatch pellets, thus enabling the production of a uniform molded product with no compositional irregularities.